Combustion of methane releases a significant amount of energy, but occurs with the concurrent evolution of one mole of the greenhouse gas CO2.CH4[g]+2O2[g]→CO2[g]+2H2O[l]ΔG°=−868.6 kJ1  [1]
A catalytic system utilizing ultraviolet light to recycle CO2 to methane increases energy production and reduces greenhouse gas emission. Cycling the system n times releases n(868.6 kJ), but evolves only one mole of CO2. (Olah et al., J. Org. Chem. (2009) 74(2):487-498; Haines, A., Medicine, Conflict, and Survival (2001) 17(1):56-62; Piver W., Environmental Health Perspectives (1991) 96:131-7; MacDonald, G. J., “The Long-Term Effects of Increasing Carbon Dioxide Levels,” Ballinger, Cambridge, Mass., 1982; Nguyen et al., Applied Catalysis A: General (2008) 335:112-120; Lo et al., Solar Energy Materials & Solar Cells (2007) 91:1765-1774; Edmonds et al., Greenhouse Gas Control Technologies, Proceedings of the International Conference on Greenhouse Gas Control Technologies, 6th, Kyoto, Japan, 2002, pp. 1427-1432; Haneda et al., Chemistry Letters (2008) 37(8):830-831; Yan et al., ACS Symposium Series 2003, 852 (Utilization of Greenhouse Gases) pp. 42-56; Lo et al., Zhongguo Huanjing Gongcheng Xuekan (2005) 15(3):143-152; Pinaeva et al., “Environmental Challenges and Greenhouse Gas Control for Fossil Fuel Utilization in the 21st Century,” “Environmental Challenges for Fossil Fuel Combustion”; “Greenhouse Gas Control and Utilization” American Chemical Society National Meeting, San Diego, Calif., 2001; Tan et al., Catalysis Today (2006) 115:269-273; Centi et al., Applied Catalysis, B: Environmental (2003) 41(1-2):143-155; Ziessel, R. “Carbon Dioxide as a Source of Carbon: Biochemical and Chemical Uses,” Aresta et al., eds. Reidel, 1987, p 113; Dey, G. R. Journal of Natural Gas Chemistry (2007) 16(3):217-226; Iijima et al., Mitsubishi Juko Giho (2002) 39(5):286-289.)
Reduction of CO2 to CH4, formally an eight-electron process, is known to occur thermally and photochemically with ultraviolet light on silica surfaces doped with transition metal carbonyls. (Gafney, H. D., “Photochemistry of Metal Carbonyls Physisorbed on Porous Vycor Glass” in “Photochemistry on Solid Surfaces” Matsuura et al., eds., Elsevier, Amsterdam, 1989, 272; Gafney et al., Inorg. Chem. (1988) 27:2815; Gafney et al., “Photosensitive Metal-Organic Systems: Mechanistic Principles and Recent Applications”, Kutal et al., eds. American Chemical Society (1993) p. 67; Brenner et al., Inorg. Chem. (1979) 18:1478-84; Brenner et al., J. Am. Chem. Soc. (1980) 102:2484-87; Simon et al., Inorg. Chem. (1988) 27:2733; Xu et al., Proc. Electrochem. Soc. (1993) 93-18, 38.)
The thermal reaction is stoichiometric, and requires high temperatures; CH4 evolution is attributed to the hydrogenation of the coordinated carbonyl. The photochemical conversion is photocatalytic, occurs at room temperature and catalytically converts atmospheric CO2 to CH4 with co-adsorbed water providing hydrogen and the reducing equivalents. (Simon, R. C. “A Study of the Photochemistry and Photocatalysis of Group VIB Hexacarbonlys adsorbed to A Porous Glass Matrix”, Ph.D. Thesis, City University of New York, 1983.)
For example, a 310-nm photolysis of W(CO)6 physisorbed onto Corning's code 7930 porous Vycor glass (PVG) or tetramethoxysilane/methanol/water (TMOS/CH3OH/H2O) xerogels leads to CH4 evolution. Photolysis initiates CO loss with the pentacarbonyl coordinating to a surface silanol group to form W(CO)5(OSi). (Simon et al., Inorg. Chem. (1985) 24:2565.) Continued photolysis leads to further CO loss, but when the complex is unable to achieve stability through coordination to the surface silanols, i.e., at an average stoichiometry of W(CO)4, the chemistry changes and the complex undergoes oxidation with the concurrent evolution of CO, H2 and CH4. (Simon, R. C. “A Study of the Photochemistry and Photocatalysis of Group VIB Adsorbed to a Porous Glass Matrix”, Ph.D. Thesis, CUNY, 1983: 94-103.)
There remains a need in the art for a method by which to convert carbon dioxide to methane which does not require high temperatures or a particular kind of light, but instead can be effected under more natural environmental conditions.